Summary We have encountered two patients who had repeated false aneurysms, seromas, and unseated grafts which presented about 18 months after aortobifemoral grafting. An inhibitor of fibroblast growth was detected in each patient's serum by in vitro dilution tests. Normal ingrowth of fibrous tissue resumed after plasmapheresis, leaving the patients clinically stable for up to 3 1/2 years after treatment. Criteria for the diagnosis of fibroblast inhibition are an unseated prosthesis which simulates infection, no bacterial growth on carefully studied cultures, and inhibition of fibroblast growth by the patient's serum in vitro. The diagnosis should be entertained in the differental diagnosis of graft infection when an unseated graft is encountered in association with a late seroma or false aneurysm. Plasmapheresis removed the inhibitor. Fibroblast inhibition has not been reported previously.